When preparing espresso coffee, a measured amount of coffee, ground to a powder of appropriate particle size, is mechanically packed down to a suitable degree in an infusion and dispensing vessel into which water, which has been heated in a boiler, is fed under pressure. The pressure of the water is kept at a level which is sufficient to cause the formation of an emulsion ("cream") between the water which passes through the packed down coffee powder and the essential oils which pass from the coffee powder into the water. If this pressure level, which is usually between 8 and 9 bar, is applied abruptly to the coffee powder, it causes a further settling of the powder inside the vessel known as the "filter", with the result that the water follows preferential routes. This means that the powder is not used uniformly since it is not fully infused and this then leads to poor coffee yield.